Abstract
Many studies have examined whether communities are structured by random or deterministic processes, and both are likely to play a role, but relatively few studies have attempted to quantify the degree of randomness in species composition. We quantified, for the first time, the degree of randomness in forest bird communities based on an analysis of spatial autocorrelation in three regions of Germany. The compositional dissimilarity between pairs of forest patches was regressed against the distance between them. We then calculated the y-intercept of the curve, i.e. the ‘nugget’, which represents the compositional dissimilarity at zero spatial distance. We therefore assume, following similar work on plant communities, that this represents the degree of randomness in species composition. We then analysed how the degree of randomness in community composition varied over time and with forest management intensity, which we expected to reduce the importance of random processes by increasing the strength of environmental drivers. We found that a high portion of the bird community composition could be explained by chance (overall mean of 0.63), implying that most of the variation in local bird community composition is driven by stochastic processes. Forest management intensity did not consistently affect the mean degree of randomness in community composition, perhaps because the bird communities were relatively insensitive to management intensity. We found a high temporal variation in the degree of randomness, which may indicate temporal variation in assembly processes and in the importance of key environmental drivers. We conclude that the degree of randomness in community composition should be considered in bird community studies, and the high values we find may indicate that bird community composition is relatively hard to predict at the regional scale.
Highlights
Understanding the processes determining the species richness, diversity, and abundance of organisms remains a key challenge
Species richness was significantly lower in the south-west region compared to the other two regions (GLMM: p#0.01; Figure 4a; detailed information in Appendix S3), and management intensity decreased species richness
The relative abundance of birds in the three regions showed a similar pattern to species richness with significant differences between years and the regions and lower abundance in the south-west (p#0.02)
Summary
Understanding the processes determining the species richness, diversity, and abundance of organisms remains a key challenge Deterministic processes such as those driven by habitat structure and heterogeneity [1,2,3], species-specific ecological traits [4,5], seasonality [6], or resource availability such as food [7], have been shown to be important in many ecosystems. Stochastic processes such as neutral dynamics [8] may, explain a proportion of the species richness and diversity of communities [9]. The role of stochastic processes in affecting bird communities has seldom been explored or quantified [12]
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